This invention relates to a method and apparatus for installing an optical fiber in a buried duct or the like.
Virtually all telecommunications carriers, such as ATandT, now employ optical fiber cable to carry telecommunications traffic between facilities. As compared to conventional copper cable, optical fiber cable affords much greater bandwidth and virtual immunity from cross talk and interference. Many of the optical fiber cables used by telecommunications carriers are buried underground both for reasons of safety and esthetics. Several different methods exist for burying optic fiber cable underground. For example, some carriers directly plow the cable underground with no protective enclosure. Other carriers first plow a plastic duct underground and then pull the optical fiber cable through the duct via a rope placed in the duct prior to burial.
Direct burial of optical fiber cable incurs several potential hazards. During burial, the plow can sometimes scrape the sheath of the optical fiber, leaving a portion of the sheath exposed. The exposed sheath portion will likely undergo electrolysis, shortening the lifetime of the cable. Direct burial also increases the risk that rocks or other objects in the ground will damage the optical fiber cable during installation.
Installing the fiber by pulling it through the duct following burial can incur difficulties as well. During burial, the duct may twist, restricting or even obstructing its interior cross-section, making it difficult to subsequently install the optical fiber cable in the duct by pulling the rope via a tractor. Unfortunately, no reliable method presently exists to detect such restrictions or obstructions before pulling the fiber through the duct. If the optical fiber encounters such a restriction, a breakaway device that connects the rope to the tractor will snap should the pulling force exceed a prescribed value, say 600 lbs. Once the breakaway device breaks or if the rope snaps, then the duct must be dug up and the restriction or obstruction removed before recommencing fiber installation. Excavation of the duct after burial is time consuming and expensive and invariably will delay fiber installation.
Thus, there is need for a technique for installing an optical fiber cable in a buried duct which overcomes the aforementioned disadvantages of the prior art.
Briefly, the present invention provides a method for installing at least one optical fiber cable through a hollow member, such as a duct or the like, having first and second open ends. A first end of the fiber is pushed into a first end of the duct to install the fiber in the duct. As the fiber is being pushed into the duct, an image of the interior of the duct is captured, typically via a lens in optical registration with the first fiber end. The fiber transmits the image for display, thereby allowing observation of any obstructions or restrictions. In case of a restriction or obstruction, means in the form of a pair of fingers extending outwardly from a housing carried at the first end of the fiber, are actuated to clear the obstruction.
The above-described method affords the advantage that the stress on the fiber incurred by pushing it through the duct is less than by pulling it through the duct as was done previously. Additionally, the present method affords the advantage of enabling installation personnel the ability to observe restrictions and/or obstructions in the duct and the ability to remove them without the need to excavate the duct after burial.